Optical interferometry is a measurement 10 technique that exploits the wave nature of light to produce extremely accurate measurements and provides excellent resolution without requiring any physical contact with the object being examined. Optical interferometry has been used to determine surface textures, shapes, distances, the speed of light through different media, and indices of refraction.
Optical interferometry is based on the phenomenon that two coherent light waves which are brought together (superimposed) behave similarly to water waves rippling through a pond. If the crest of one wave coincides with the crest of another wave, the waves reinforce one another in what is referred to as constructive interference. If the crest of one wave coincides with the trough of another wave, the waves cancel each other out. This canceling process is referred to as destructive interference. Several wave disturbances arriving at a point simultaneously result in a disturbance that is the vector sum of each of the separate disturbances.
The Michelson interferometer is a well known device that uses interferometry to make extremely precise measurements. One common embodiment of this device, shown in FIG. 1A includes a partially mirrored surface 200, which serves as a beam splitter, to divide a beam of monochromatic light into two beams 204 and 208 that are directed to travel in different directions. Monochromatic light is light having one color, and hence, one wavelength. One divided beam 204 reflects off a flat reference mirror 206 back to the beam splitter. The other divided beam 208 reflects off of a surface 210 being studied and returns back to the beam splitter where the two divided beams are recombined in an output beam 212. Recombining optical waves that are out of phase partially or totally cancel one another out. Optical waves that are in phase reinforce each other. The combined beams produce a pattern of alternating light and dark regions known as an interference pattern.
The difference in the lengths of the overall paths taken by each of the split beams is encoded in the interference pattern. For example, a difference between the distance traversed by the two beams equal to one wavelength (one-half wavelength up and back) results in the recombined wave going through one bright-dark-bright cycle. A dark region is created whenever the round-trip path along one arm, or beam path, of the interferometer increases or decreases by one-quarter wavelength with respect to the other arm since the total increased distance traveled by one of the split beams both up and back is one-half the wavelength (two times one-quarter wavelength) of the monochromatic beam.
Because the resolution of the Michelson interferometer is approximately one-half of the wavelength of the monochromatic light beam, optical interferometry provides very precise distance measurements.
Another type of interferometer is a Mach-Zehnder interferometer, shown in FIG. 1B. A Mach-Zehnder interferometer typically uses light 214 from a coherent light source that is split into two optical beams 216 and 218. The first split, or divided, beam can be used as a reference and traverses an optical path of fixed length. The other divided beam can be guided along an optical path having a variable length. The beams are subsequently recombined to produce an output beam 220 having an interference pattern. The length of the variable optical path length may be lengthened or shortened to achieve a desired relation between the two beams.
Optical interferometry has many practical applications. In the field of quality control, optical interferometry is used to measure distances and surface textures. Techniques for measuring surface texture using interferometry are described in Robinson, G.M., et al., "Optical Interferometry of Surfaces," Scientific American, pages 66-71, July 1991. The techniques described in that article are particularly suitable for measuring the surface textures of products such as photographic film, magnetic tape, and computer diskettes. Interferometric techniques that measure surface texture are also used to measure the degree of wear on products such as bearings.
Optical measurement techniques are well suited to physical dimension measurements in which accuracies on the order of microns are required. One common technique for optical measurements of physical dimensions is optical triangulation. Optical triangulation involves directing two laser beams from an electro-optic sensor unit towards a surface of interest, with the laser beams originating at points separated by several inches, and intersecting at the surface. As the distance between the sensor unit and surface of interest changes, the angle of one laser beam must be changed to keep the beam intersection point at the surface. The angle of this laser beam therefore provides an indication of the distance between the sensor head and surface of interest.
If two sensor heads are positioned on opposite sides of an object, the thickness of the object is determined by subtracting the distance readings of each sensor from the known separation between the two sensor heads. Thickness measurements by optical triangulation thus requires that a sensor be located on each side of an object under test. However, many structures having a thickness to be measured are formed on supporting structures, thus precluding access to one side of such structures. Therefore, optical triangulation is not suitable for applications lacking access to both sides of such structures.
Present optical triangulation systems can attain distance measurement accuracies of .+-.0.0001 inches (.+-.2.5 microns). When two such devices are used to determine thickness, the accuracy limits of each sensor combine to yield a total thickness accuracy limit of .+-.0.00014 inches (.+-.3.5 microns). However, there are plastic product manufacturing applications where greater thickness accuracies, up to .+-..00003 (.+-.1 .mu.m), are required.
There are many other applications where it is necessary to measure the thickness of optically transmissive (transparent and translucent) media, associated with, for example, lenses, video tape, audio tape, photographic films, coatings, compact disks, laminated structures, and supported structures. However, optical triangulation methods cannot measure the thicknesses of transparent or translucent objects because the sensor requires a visible illuminated spot, from diffuse reflection of the irradiating beams, on the surface of interest. This spot is measured by an optical sensor and as the surface distance changes, the beam angle of one of the beams is changed to maintain a single spot. However, transparent or translucent surfaces do not reflect enough light to provide a detectable spot.
Therefore, there is a need for a system capable of measuring the thickness of optically transparent or translucent media. A further need exists for a thickness measurement system that can be used when only one side of the object having the thickness to be measured is accessible. Still a further need exists for a thickness measurement system and method that can determine thicknesses of optically transmissive media with accuracies greater than .+-.0.00014 inches (.+-.3.5 microns).